1. Field of the Invention
The present invention relates to a semiconductor memory device, and more particularly, to a NAND flash memory device.
2. Description of the Related Art
A semiconductor memory device is a memory device which can store data and read it out when necessary. Semiconductor memory devices may be roughly classified into a Random Access Memory (RAM) and a Read Only Memory (ROM). The RAM is a volatile memory device, data stored in which disappears upon power off, whereas the ROM is a nonvolatile memory device, data stored in which does not disappear despite power off.
The RAM includes a Dynamic RAM (DRAM) and a Static RAM (SRAM), while the ROM includes a Programmable ROM (PROM), an Erasable PROM (EPROM), an Electrically EPROM (EEPROM), and a flash memory.
The flash memory, though it is a nonvolatile memory, is capable of programming and erasure in the on-line state. Like the EEPROM, the flash memory uses the electrically erasing technology, and its entire data may be erased in one or several seconds. Data stored in the flash memory chip may be erased at a time or on a block basis, but may not be erased on a byte basis. The flash memory may store modifiable control programs, and may be used as a substitute for a secondary memory.
The flash memory is classified into a Negated ‘AND’ (NAND) flash memory and a Negated ‘OR’ (NOR) flash memory. The NOR flash memory may easily constitute a circuit with processors and the like, since it uses the SRAM or ROM-type interface scheme. In contrast, the NAND flash memory is complex in the interface scheme, compared with the NOR flash memory. However, the NAND flash memory is advantageous over the NOR flash memory in terms of integration and price.
Recently, the NAND flash memory has become popularly used as a secondary memory of a system due to its advantage that information stored therein is not erased even in the power-off state. However, sometimes information stored in the NAND flash memory may be damaged by power off of the system or an unscheduled outage.
The damaged information in the NAND flash memory may be correctable by an Error Check Code (ECC). However, if the memory is damaged in bulk in units of pages, the information may not be recovered.
Therefore, a method for protecting data in the NAND flash memory upon power off or an unscheduled outage has long been required.